Conditional probability

A conditional probability is the probability of an event, given some other event has already occurred. In the below example, there are two possible events that can occur. A ball falling could either hit the red shelf (we'll call this event A) or hit the blue shelf (we'll call this event B) or both.

If we know the statistics of these events across the entire population and then were to be given a single ball and told "this ball hit the red shelf (event A), what's the probability it also hit the blue shelf (event B)?" we could answer this question by providing the conditional probability of B given that A occurred or P(B|A).